Biology in the News Explained

The definition of “drought”

The western U.S. has been under “drought” conditions on and off for the last ten years or so. Why is “drought” in quotes? Because drought is a relative condition, referring to less rain than normal – a level of rain considered drought in the Alaka`i Swamp would cause major flooding in Tucson.

So what is really going on in the West? The quick answer is that we probably don’t have enough data really to know. Ten years of dry conditions is a blink in the long term. The ultimate question is if the current drier trend is something that will continue in the long term, or is really just a blip, and the truth is that no one can know for sure.

But what we can know about is the long term climate of the past. A panel of scientists analyzing conditions in the southwest noted the following (from a NY Times article [sub. req'd]):

…the water allocation agreement for the basin, the Colorado River Compact, was negotiated in 1922 based on river flow records dating to the 1890s, when gauging stations were established. The agreement assumed that the annual river flow was 16.4 million acre feet — enough to cover 16.4 million acres to a depth of one foot.

But for some time, the panel said, researchers have known that the early 20th century was unusually wet and that 15 million acre feet was a more accurate estimate of the flow. Recent studies based on tree rings put the figure lower still — as low as 13 million acre feet — and suggest that “drought episodes are a recurrent and integral feature of the region’s climate.”

The harsh reality is apparently that over the last couple of centuries at least, the typical amount of water in the west jibes pretty closely with conditions we are seeing more recently, and the period when the west became heavily settled coincides with an unusual wet spell, combined with technology (dams, etc.) that allowed people to use more of the water that is there.

The long term implications for this trend are far-reaching. Biologically, the landscape of the west has been irreparably altered by the introduction of dams and cattle. Both of these in turn have facilitated establishment by lots of invasive weeds that are massively altering the landscape further. Two examples: Salt cedar and Russian olive have taken over many riparian areas in the southwest, and are spreading north (probably helped by human-enhanced climate change). Salt cedar not only crowds out native plants and lowers the water table, but its excretion of salt changes the chemistry of the soil, making restoration of these areas especially difficult. Native riparian trees such as willows can compete with salt cedar under the natural cycle of floods, but thousands of dams (built to provide water and power to an increasingly unsustainable western human population) have disrupted this cycle, under which conditions salt cedar easily takes over, disrupting the entire ecology of the system, because so many animals, vertebrate and invertebrate, depend on the native willows.

Cheatgrass is an invasive grass that produces intense fires that occur much more often than the normal fire cycle to which the animals and plants of the Great Basin are adapted. Neither are cows, so in addition to drought itself, cheatgrass has gotten a lot of attention because it impacts “traditional” ranching in the area.

But the effects of the West suddenly finding itself quite overpopulated given the amount of water we can expect in the near future reach into the sociological as well. As one example of many, our local Women’s Resource Center, which provides support primarily to victims of domestic violence, has to gear itself up every summer for a big run on its services; the drier the summer, the more domestic abuse, presumably because of family stress about financial problems.

The bottom line is that at the very least, the level and methods of ranching and agriculture that people have become accustomed to over the last few generations in areas defined as desert, based on their low rainfall, is no longer sustainable. There are different ways of ranching cattle that can significantly reduce problems associated with overgrazing, but people are slow to change. If we are not actually experiencing a “drought,” but rather emerging from a wet period, dark times are ahead for rural western economies, because the cities will be grabbing the resources to sustain millions of people living in the desert. Forget about the cows…which if you are an ecologist such as myself, would be a silver lining in all of this, if the native ecosystems that are left weren’t going to disappear along with them.


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